It is known to chemically modify catalytically active zeolite surfaces by silylation with organosilanes. Prior art patents relating to the silylation of zeolite surfaces are, e.g., U.S. Pat. Nos. 4,390,414 and 4,451,572 to Ian A. Cody. These patents disclose the silylation of zeolite to form catalysts useful for hydrodewaxing. The organosilane selected for silylation of a given zeolite, as described by these references, is one capable of entering into the pores of the zeolite to chemically react with available sites within the pores, as well as with hydroxyl sites on the external, or peripheral surface of the treated zeolite. To generate a surface which can be silylated, and which is also catalytically active, the zeolite is ion exchanged with a decomposable ion, e.g., NH.sub.4.sup.+, then calcined in air or inert atmosphere to convert the NH.sub.4.sup.30 sites of the zeolite to H.sup.+ sites. The surface of the H.sup.+ zeolite, inclusive of the internal surface, or surface within the pores of the zeolite, and the external, or peripheral surface of the H.sup.+ zeolite, is then silylated by contact at elevated temperature and anhydrous conditions with the organosilane. The chemically modified zeolite may then be loaded with a catalytically active metal hydrogenation component, or with said catalytically active metal hydrogenation component prior to silylation of the H.sup.+ zeolite. At Columns 2 and 3 of each of the patents, there is given a brief review of prior art patents relating to the silylation of zeolites.
Organosilanes, in accordance with the teaching of U.S. Pat. Nos. 4,390,414 and 4,451,572, supra, react with certain H.sup.+ zeolites by sequestering hydroxyl sites within the structure, this generating a smaller pore hydrophobic structure. The interior zeolitic sites are thus silylated, along with zeolytic hydroxyl groups on the external surface of the zeolite structure. It therefore occurred to us that a different zeolite catalyst for different uses could be formed by selectively silylating only the external surface, without silylating the internal surface, of the zeolite.